ANHARMONIC TEMPERATURE FACTORS, ANOMALOUS-DISPERSION EFFECTS AND BONDING CHARGES IN GALLIUM-ARSENIDE

The introduction of dispersion corrections in the generalized structure-factor expression for the zinc blende structure leads to the breakdown of Friedel's law for all even-index reflections, owing to bonding or anharmonicity in these crystals. A systematic and precise X-ray diffraction intensity measurement has been carried out using a spherical single crystal of gallium arsenide at 300 K. Least-squares refinement of the experimental data yields the Debye-Waller factors B(Ga) = 0.62 (2), B(As) = 0.49 (2) angstrom 2 for the harmonic model and B(Ga) = 0.670 (3), B(As) = 0.470 (2) angstrom 2 and individual anharmonic thermal factors beta(Ga) = 3.15 (59) x 10(-18) and beta(As) = 8.92 (182) x 10(-18) J angstrom -3 for the anharmonic model. The residual index for the discrepancy between measured and calculated structure factors in the present work is R = 1.237% for the harmonic model and R = 1.229% for the anharmonic model. The present work reveals excellent agreement in both sign and magnitude of the Bijvoet inequalities for a large number of reflections. Covalent charge transfer has also been deduced from the experimental measurements on a few quasi-forbidden Bragg reflections showing a net transfer of charge from Ga to As.